1. Field of the Invention
The invention relates to a zinc sulfide body for optical purposes produced by the CVD (Chemical Vapor Deposition) technique.
2. Discussion of Prior Art
Zinc sulfide bodies of this type, their production and their properties are described in Proc. Symp. Mater. Sci. Aspects Thin Film Syst. Sol. Energy Convers. 1974, pp 402-418, for example. Such bodies, produced mostly in plate or dome form, have high transmission in the 8 to 12 micron region and are used for infrared windows, for example. The transmission of such zinc sulfide bodies in the visible region is low. In the infrared region the exhibit a pronounced absorption band at about 6 microns.
The invention has as its object to improve a zinc sulfide body for optical purposes produced by the CVD technique in such a way that it has higher transmission in the visible region and more uniform transmission in the infrared region than the prior-art zinc sulfide bodies.
The zinc sulfide body in accordance with the invention is characterized in that it has been subjected to an aftertreatment involving the concurrent use of a gas or gas mixture, under high pressure, which does not react with zinc sulfide and of a temperature of at least 300.degree. C., and that it exhibits no absorption band at about 6 microns.
Such a zinc sulfide body which has been subjected to an aftertreatment involving the concurrent use of a gas or gas mixture, under a pressure ranging from 800 to 3000 bars, and preferably from 1000 to 3000 bars, which does not react with zinc sulfide, and of a temperature ranging from 600.degree. to 1200.degree. C., and preferably from 850.degree. to 1000.degree. C., has proved itself particularly well.
It has been found that an aftertreatment involving the concurrent use of gas or gas mixture, under a pressure of 1200 bars, which does not react with zinc sulfide, and of a temperature of 900.degree. C. is particularly advantageous.
Especially the inert gases have proved themselves as gases which will not react with zinc sulfide. The preferred gas is argon. A gas particularly contemplated too is nitrogen.
The duration of the pressure-temperature treatment depends on the pressure and temperature levels. While at 1200 bars and 900.degree. C., for example, a treating time of 4 hours is required to obtain a zinc sulfide body having the desired properties, the treating time at 2000 bars and 1100.degree. C. is reduced to one hour. Generally the treating time will decrease with increase of pressure and/or in temperature.
Surprisingly, the aftertreated zinc sulfide bodies in accordance with the invention exhibit higher transmission in the visible region than the known untreated bodies, and no absorption band at about 6 microns in the infrared region.
Like untreated zinc sulfide bodies, the zinc sulfide bodies in accordance with the invention are suitable for use as infrared windows.